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| Causes of Cancer |
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| A chemical or other environmental agent that produces cancer is called a carcinogen. The world Health organisation estimates that carcinogens may be associated with 60 - 90% of all human cancer. |
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| Depending upon their mode of action carcinogens are of 3 types. |
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| (1) Carcinogens that cause alternations in the genetic material (DNA) or genes. This results in oncogenic transformation that leads to cancer. For e.g., radiations and chemicals. Ultraviolet (UV) light from the sun. For e.g., may cause genetic mutations in exposed skin cells and lead to cancer. The hydrocarbons found in cigarette tar are carcinogenic and most of the lung cancer patients are smokers. |
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| (2) Carcinogens that promote proliferation of cells, which have already undergone genetic alterations. These are called Tumour promoters. For e.g., some growth factors and hormones. |
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| (3). Cancer causing DNA and RNA viruses called Tumour viruses are responsible for Oncogenic transformation. |
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| Some of the genes, called oncogenes, have the ability to transform a normal cell into a cancerous cell. Such an oncogenic transformation requires activation of the oncogenes by carcinogens. |
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| Normal cell growth is under the control of genes, which regulate cell proliferation, differentiation and survival. Alterations in these genes lead to oncogenic transformation. Cancer is caused by mutant alleles of these genes whose products do not conform to normal regulation in cell growth and proliferation. As a result they proliferate uncontrollably forming tumours. |
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